It is well understood that air pressure progressively reduces as altitude increases. The consequence for internal combustion engines is that power and torque output also reduces so that a vehicle driver may notice a loss of performance in mountain terrain. Means of providing for loss of performance, for example supercharging, are well understood, but generally impose a significant cost penalty.
Development of vehicles is tending towards engines of smaller swept volume, but relatively higher maximum power and torque. This can be achieved to some extent by turbocharging. A well known phenomenon of an exhaust turbocharger is turbo ‘lag’, whereby a delay may be experienced by the vehicle driver between demanding an acceleration, and sensing a corresponding increase in engine output torque.
The turbo lag effect is more acute at altitude because the mass flow of gas through the engine is reduced (absent some special corrective measures), with the consequence that the turbocharger turbine takes longer to spool up to a speed at which the turbocharger compressor becomes effective. The turbo lag effect may also be exacerbated by the use of smaller engines.
Hybrid vehicles typically comprise an internal combustion engine, an electric motor and a battery. A so-called plug in hybrid includes a traction battery which may be charged overnight from a relatively low cost domestic electric supply. Electric traction is not sensitive to altitude.
One kind of hybrid vehicle is a sequential parallel hybrid whereby electric traction is used in preference to the internal combustion engine until the remaining charge stored in the battery falls to a pre-determined minimum value.
A parallel hybrid vehicle may also use the internal combustion engine and electric motor together to boost acceleration, and this technique may be used to compensate for turbo lag.
However in normal use as a sequential parallel hybrid, i.e. electric traction is used preferentially, there may be insufficient reserve of electrical power in the vehicle to boost an internal combustion engine at the minimum state of charge of the battery. On the other hand, the benefits of a parallel hybrid vehicle cannot be fully realised if a greater reserve of electrical power is retained in case of changing conditions of vehicle use, in particular to deal with all circumstances in which an electric and engine mode of operation may be required.
It is against this background that the present invention has been conceived. Embodiments of the invention may provide an improved method or apparatus which addresses one or more of the above issues. Other aims and advantages of the invention will become apparent from the following description, claims and drawings.